Sclerotinia infects over 100 species of plants, including numerous economically important crops such as Brassica species, sunflowers, dry beans, soybeans, field peas, lentils, lettuce, and potatoes (Boland and Hall, 1994). Sclerotinia sclerotiorum is responsible for over 99% of Sclerotinia disease, while Sclerotinia minor produces less than 1% of the disease. Sclerotinia produces sclerotia, irregularly-shaped, dark overwintering bodies, which can endure in soil for four to five years. The sclerotia can germinate carpogenically or myceliogenically, depending on the environmental conditions and crop canopies. The two types of germination cause two distinct types of diseases. Sclerotia that germinate carpogenically produce apothecia and ascospores that infect above-ground tissues, resulting in stem blight, stalk rot, head rot, pod rot, white mold, and blossom blight of plants. Sclerotia that germinate myceliogenically produce mycelia that infect root tissues, causing crown rot, root rot and basal stalk rot.
Sclerotinia causes Sclerotinia stem rot, also known as white mold, in Brassica, including canola. Canola is a type of Brassica having a low level of glucosinolates and erucic acid in the seed. The sclerotia germinate carpogenically in the summer, producing apothecia. The apothecia release wind-borne ascospores that travel up to one kilometer. The disease is favoured by moist soil conditions (at least 10 days at or near field capacity) and temperatures of 15-25° C., prior to and during canola flowering. The spores cannot infect leaves and stems directly; they must first land on flowers, fallen petals, and pollen on the stems and leaves. Petal age affects the efficiency of infection, with older petals more likely to result in infection (Heran, et al., 1999). The fungal spores use the flower parts as a food source as they germinate and infect the plant.
The severity of Sclerotinia in Brassica is variable, and is dependent on the time of infection and climatic conditions (Heran, et al., 1999). The disease is favored by cool temperatures and prolonged periods of precipitation. Temperatures between 20 and 25° C. and relative humidities of greater than 80% are required for optimal plant infection (Heran, et al., 1999). Losses ranging from 5 to 100% have been reported for individual fields (Manitoba Agriculture, Food and Rural Initiatives, 2004). On average, yield losses equal 0.4 to 0.5 times the percentage infection. For example, if a field has 20% infection (20/100 infected plants), then the yield loss would be about 10%. Further, Sclerotinia can cause heavy losses in wet swaths. Sclerotinia sclerotiorum caused economic losses to canola growers in Minnesota and North Dakota of 17.3, 20.8 and 16.8 million dollars in 1999, 2000, and 2001, respectively. (Bradley, et al. 2006)
The symptoms of Sclerotinia infection usually develop several weeks after flowering begins. The plants develop pale-grey to white lesions, at or above the soil line and on upper branches and pods. The infections often develop where the leaf and the stem join because the infected petals lodge there. Once plants are infected, the mold continues to grow into the stem and invade healthy tissue. Infected stems appear bleached and tend to shred. Hard black fungal sclerotia develop within the infected stems, branches, or pods. Plants infected at flowering produce little or no seed. Plants with girdled stems wilt and ripen prematurely. Severely infected crops frequently lodge, shatter at swathing, and make swathing more time consuming. Infections can occur in all above-ground plant parts, especially in dense or lodged stands, where plant-to-plant contact facilitates the spread of infection. New sclerotia carry the disease over to the next season.
Conventional methods for control of Sclerotinia diseases include (a) chemical control, (b) disease resistance and (c) cultural control, each of which is described below.
(a) Fungicides such as benomyl, vinclozolin and iprodione remain the main method of control of Sclerotinia disease (Morall, et al., 1985; Tu, 1983). Recently, additional fungicidal formulations have been developed for use against Sclerotinia, including azoxystrobin, prothioconazole, and boscalid. (Johnson, 2005) However, use of fungicide is expensive and can be harmful to the user and environment. Further, resistance to some fungicides has occurred due to repeated use.
(b) In certain cultivars of bean, safflower, sunflower and soybean, some progress has been made in developing partial (incomplete) resistance. Partial resistance is often referred to as tolerance. However, success in developing partial resistance has been very limited, probably because partial physiological resistance is a multigene trait as demonstrated in bean (Fuller, et al., 1984). In addition to partial physiological resistance, some progress has been made to breed for morphological traits to avoid Sclerotinia infection, such as upright growth habit, lodging resistance and narrow canopy. For example, bean plants with partial physiological resistance and with an upright stature, narrow canopy and indeterminate growth habit were best able to avoid Sclerotinia (Saindon, et al., 1993). Early maturing cultivars of safflower showed good field resistance to Sclerotinia. Finally, in soybean, cultivar characteristics such as height, early maturity and great lodging resistance result in less disease, primarily because of a reduction of favorable microclimate conditions for the disease. (Boland and Hall, 1987; Buzzell, et al., 1993)
(c) Cultural practices such as using pathogen-free or fungicide-treated seed, increasing row spacing, decreasing seeding rate to reduce secondary spread of the disease, and burying sclerotia to prevent carpogenic germination may reduce Sclerotinia disease but not effectively control the disease.
All Canadian canola genotypes are susceptible to Sclerotinia stem rot (Manitoba Agriculture, Food and Rural Initiatives, 2004). This includes all known spring petalled genotypes of canola quality. There is also no resistance to Sclerotinia in Australian canola varieties. (Hind-Lanoiselet, et al., 2004) Some varieties with certain morphological traits are better able to withstand Sclerotinia infection. For example, Polish varieties (Brassica rapa) have lighter canopies and seem to have much lower infection levels. In addition, petal-less varieties (apetalous varieties) avoid Sclerotinia infection to a greater extent (Okuyama, et al., 1995; Fu, 1990). Other examples of morphological traits which confer a degree of reduced field susceptibility in Brassica genotypes include increased standability, reduced petal retention, branching (less compact and/or higher), and early leaf abscission. Jurke and Fernando, (2003) screened eleven canola genotypes for Sclerotinia disease incidence. Significant variation in disease incidence was explained by plant morphology, and the difference in petal retention was identified as the most important factor. However, these morphological traits alone do not confer resistance to Sclerotinia, and all canola products in Canada are considered susceptible to Sclerotinia. 
Winter canola genotypes are also susceptible to Sclerotinia. In Germany, for example, no Sclerotinia-resistant varieties are available. (Specht, 2005) The widely-grown German variety Express is considered susceptible to moderately susceptible and belongs to the group of less susceptible varieties/hybrids. (See, Table 4)
Spraying with fungicide is the only means of controlling Sclerotinia in canola crops grown under disease-favorable conditions at flowering. Typical fungicides used for controlling Sclerotinia on Brassica include Rovral™ from Bayer and Ronilan™/Lance™ from BASF. The active ingredient in Lance™ is Boscalid, and it is marketed as Endure™ in the United States. The fungicide should be applied before symptoms of stem rot are visible and usually at the 20-30% bloom stage of the crop. If infection is already evident, there is no use in applying fungicide as it is too late to have an effect. Accordingly, growers must assess their fields for disease risk to decide whether to apply a fungicide. This can be done by using a government provided checklist or by using a petal testing kit. Either method is cumbersome and prone to errors. (Hind-Lanoiselet, 2004; Johnson, 2005)
Numerous efforts have been made to develop Sclerotinia resistant Brassica plants. Built-in resistance would be more convenient, economical, and environmentally-friendly than controlling Sclerotinia by application of fungicides. Since the trait is polygenic it would be stable and not prone to loss of efficacy, as fungicides may be.
Spring canola (Brassica napus subsp. oleifera var. annua) differs from winter canola (Brassica napus subsp. oleifera var. biennis) primarily in the absence of an obligate vernalization requirement. Asiatic rapeseed, and canola versions, have a low to intermediate requirement for vernalization, and are known as semi-winter types. While winter canola cannot finish its reproduction cycle when planted in the spring, early spring planting and exposure to cold enables Asiatic material to flower and set seed. Asiatic material cannot finish its reproduction cycle if planted in late spring. In controlled conditions, winter material requires 12-14 weeks of vernalization while Asiatic material requires 2-8 weeks. Table 1 summarizes the differences between winter, semi-winter (Asiatic) and spring canola varieties.
TABLE 1Main determinations of growth habit in Brassica napus materialsTypeSpring*SpringSemi WinterWinter(Asiatic)GrowingCanada,AustraliaChina,EuropeareasEuropeJapanVernalizationNoneNone2-8 weeks12-14 weeksRequirementintermediatestrong or fullTime ofSpringFallFallFallseeding(Increasing(Decreasing(Decreasing(DecreasingDay Length)Day Length)Day Length)Day Length)Number of30-9090-150120-180150-270days untilflowering*Canadian, European and Australian spring materials can be planted and grown in any environment or seeding time for spring canola.
Some Chinese (semi-winter) cultivars of rapeseed/canola are partially resistant to Sclerotinia. For example, ChunYun, et al., 2003; HanZhong, et al., 2004; XeiXin, et al., 2002; YongJu et al., 2000; ChaoCai, et al., 1998 describe partially resistant varieties of rapeseed. However, some of these varieties are not canola quality, and all of them require vernalization.
The partial field resistance in Chinese varieties originated mostly from the rapeseed variety Zhong you 821. Despite improvements in partial resistance in Zhong you 821, its reaction to disease is less stable under environmental conditions favorable for development of Sclerotinia (Yunchang, et al., 1999). This indicates a lower level of partial resistance (Li, et al., 1999).
Some Japanese cultivars of rapeseed have partial stem resistance to Sclerotinia. Partial stem resistance was detected by indoor tests in comparison with winter canola (Brun, et al., 1987). However, these varieties are not canola quality and are semi-winter types (see, Table 1).
Breeding for Sclerotinia field resistance in canola has been very difficult due to the quantitative nature of this trait. Further, the incorporation of physiological resistance with morphological traits that avoid or reduce infection multiplies the complexity of breeding for resistance. In addition, it has been very difficult to screen for resistance because of the direct environment interaction (i.e., temperature and humidity requirements, as well as microenvironment requirements) with the plant population. As stated above, there are no Canadian spring Brassica varieties with resistance to Sclerotinia, this despite many years of co-evolution and environmental pressure to select for this trait. The highest available level of field resistance in rapeseed (and recently some canola materials) was attained via breeding efforts in China as described with Zhong you 821 (Yunchang, et al., 1999). The levels of such partial resistance or tolerance are relatively low as fungicide applications are still recommended on all rapeseed and canola materials in China (verbal communication) (Baocheng, et al., 1999). Clearly, Brassica and canola varieties with high levels of resistance to Sclerotinia are not found in nature.
Canola quality Brassica napus was developed in the 1970's. Despite 30 years of extensive breeding efforts, no canola varieties resistant to Sclerotinia have previously been developed. The breeding efforts included quantitative trait loci analysis (Zhao-Jianwei, et al., 2003), mutagenesis breeding (Mullins, et al., 1999; Wu-Yanyou, et al., 1996; LiangHong, et al., 2003), extensive screening efforts (Sedune, et al., 1989; Zhao, et al., 2004); and screening for expressed sequence tags (ESTs) (Rugang, et al., 2004), to name a few. Several spring canola varieties with moderate tolerance to Sclerotinia have been developed (Ahmadi, et al., 2000a; Ahmadi, et al., 2000b; BaoMing, et al., 1999; and Liu, et al., 1991), however the level of tolerance is low and the lines cannot withstand high disease pressure. Recently, transgenic canola has been developed carrying an oxalic oxidase gene (U.S. Pat. No. 6,166,291 and divisional patents thereof); however there are regulatory and social issues associated with transgenic plants. Winter canola genotypes with resistance to Sclerotinia are also needed as indicated by fungicide applications (Johnson, 2005). Accordingly, significant technical human intervention is required to breed canola varieties that are resistant to Sclerotinia. 